I need an idea of why a family would need to move states. But a specific idea. Like if they move for a job, what kind of job? Or another reason to move would be helpful besides for a job.

My dad was a Lutheran pastor, so we moved three times (as he changed parishes) twice in one state and then to a northern state.

My grandfather was a farmer and moved from Nebraska where he grew corn and wheat to Idaho (the government opened up cheap homesteads) where he grew potatoes and soybeans.

Work, school, grandparents need assistance, wanting to be in a "prettier" state (mountains, ocean, big city), retirement

How about being closer to their family. Or, another reason to move is to attend a university.

Army kids move a lot.

I'm not sure why you want us to be specific, this is your book, use your imagination. That's what writing is about. If you're having a hard time even coming up with a reason the family in your story are going to move, you'll have a heck of a time with the rest of the story.

Maybe a parent got a job opportunity, or they want to go somewhere new for a "fresh start," or there was a divorce/separation. The possibilities are endless!

Warmer climate (old age, injury hurts in cold weather)
Cheaper housing after losing a job or business
Be near a grown child, parents, grandparents
Returning to home town
Oil and gas jobs in South Dakota (fracking)